Two particles A and B having charges of+ 200*10^-6 and -400*10^-6C respectively are held fixed at a separation of 20 cm. Locate the points on the line AB where (a) the electric field is zero (b) the electric potential is zero. Answer - 48.3cm , 20cm. How?

in case of oppositely charged particles the null point shud be taken outside the charge separation on the same line or axis on which the carges are placed and dat too on the side of the charge with minimum magnitude of the two charges......take an arbitrary point p near  A outside  the charges separation..... let its distnce from A be x  and dat from B be 20+x.

calculate the electric field on the point due to both the charges nd equate them..... solve for x to get the field

for potential take the point in between the two charges.... repeat the same procedure taking only magnitudes in considerations not the signs.... nd calculate....